1. Field of the Invention
The present invention relates to a liquid level detecting device for optically detecting the liquid level in a tank or the like containing water, oil or the like therein and a method for producing the same.
2. Description of the Prior Arts
Heretofore, U-shaped glass sensors have been employed as a sensor for optically detecting a position of water level or oil level.
This sensor is prepared by bending the middle portion of a glass rod having a diameter of several mm. And the sensor is utilized in such a way that the U-shaped portion of the sensor is placed at a prescribed level in a liquid tank, light is propagated from one end of the glass rod, and the light thus propagated is received by the other end of the glass rod, whereby a difference of light transmission loss between the case where the U-shaped portion of the glass rod is in atmosphere and the case where the U-shaped portion is in a liquid is measured, so that it is determined whether the liquid level is above the position where the sensor is placed or not. Namely, in this glass rod, light behaves in response to refractive index of a material encompassing the glass rod, and light components each having a large angle of incidence in a curved portion project outside the glass rod, so that it results in increase of light transmission loss.
In such a type of sensor, since the light transmission loss thereof is very large, a detection sensitivity of the sensor is low, besides a length of the glass rod in its straight line part cannot be so prolonged, so that it requires such construction that a light source and a photoreceptive element are disposed very close to the U-shaped detecting part. As a result, there causes such difficulty that a detection signal cannot be transmitted to a control center if such detection signal has not at once been converted to an electrical signal in the vicinity of the detecting part. Furthermore, if an optical signal is intended to transmit to the control center, it is required to connect the glass rod to bundle fibers. In order to comply with such requirement, however, connectors are separately required for the glass rod and the bundle fibers, which involves the problem that the optical signal attenuates in such connecting portions.
In addition, since the light source and the photoreceptive element are fixedly disposed on a tank containing a material to be detected, there is such a disadvantage that maintenance and inspection of such device are troublesome.
In order to select an angle of incidence of the light to be fed to the glass rod so that the light transmission loss thereof becomes small, it is necessary to establish a bending radius of the glass rod with a high precision, and further a considerable strength is also required for maintaining the U-shaped form of the glass rod. Consequently, there is also such a disadvantage that miniaturization of such device is difficult.
Still further, there is also a disadvantage that when a size of the sensor becomes large and the sensor is positioned above the liquid level, a build-up in a material to be detected accrues on the sensor, so that an amount of leakage of light increases above the liquid level, and it brings about further lowering of detecting sensitivity.
On the one hand, relatively recently, an optical fiber, e.g., as disclosed in U.S. Pat. No. 4,083,625, consisting of a core made of a transparent material and a cladding layer covering the core therewith and made of another transparent material having a lower refractive index than that of the former transparent material forming the core has widely been utilized particularly in a field of information transmission as an optical guide having a very thin outer diameter, besides possessing a small transmission loss.
When such an optical fiber is bent in U-shaped form as in the case of the above-mentioned glass rod, the optical fiber can be utilized as a liquid level detecting device. In this case, however, the optical fiber is required to have a considerably long length as compared with the outer diameter thereof, so that the whole size of a device obtained from the optical fiber cannot be so miniaturized. Besides, since the optical fiber is very liable to be broken, there is a disadvantage that if the optical fiber has been bent into U-shaped form with a small curvature radius, it is easily broken.
U.S. Pat. No. 4,083,625 discloses an invention relating to an optical fiber junction device prepared by disposing a pair of optical fibers in side-by-side relationship, abutting an end face of another optical fiber upon end faces of the pair of optical fibers, and fusing the abutted portion together, thereby shaping a Y-shaped joined portion. However, it is to be noted that this joined portion relates to a mere Y-shaped branched, joined portion of optical fibers, but it does not function as a liquid level detecting device.